Instrument clusters used in automobiles commonly use microprocessors to measure the vehicle speed signal and display the speed on an air-core gauge. Two common measurement techniques are used in the prior art. One, a pulse period measurement technique uses a high-resolution timer to store the time when each speed pulse edge occurs. The pulse period is the difference in 2 consecutive values. The high-resolution timer typical in current instrument clusters are 16-bits with a 2 microsecond resolution. The maximum period that can be measured without considering overflows is: EQU Max Period=$10000*2 usec=131.072 milliseconds
If overflows are considered, then the period is a relationship between the time difference and the number of overflows accumulated. An overflow is a count of how many times the timer has overflowed from $FFFF to $0000. Using an overflow counter allows longer pulse periods to be measured. Pulse period measurement has the advantages of high measurement accuracy and fast response, but exhibits a "steppy" response at low speeds and a perceived pause upon signal loss.
A second measurement technique counts the number of pulses per time interval. A timer provides a time base for counting speed pulses. Each speed pulse count is placed into a finite length buffer and periodically summed yielding a frequency. This frequency is then converted to vehicle speed. While this technique produces a smooth response over the entire range, the resolution of this method requires extensive filtering to achieve acceptable values, severely affecting the response.